Artificial window size interrupt reduction system for CDMA receiver

ABSTRACT

A method and an apparatus for searching code division multiple access pilot signal energies includes limiting a pilot signal window size above a lower limit and only analyzing search result data corresponding to an instructed pilot signal window size. A central processing unit instructs a searcher receiver within a mobile station modem application specific integrated circuit to search for pilot signal energies within an instructed pilot signal search window unless the instructed pilot signal window size is smaller than a pilot signal window size lower limit. In such a case, the pilot signal search window size is artificially designated as the pilot signal window size lower limit to prevent excessive search completion interrupts from over utilizing central processing unit resources. The central processing unit analyzes only the middle search results corresponding to the pilot signal window size as instructed from the base station to maintain the ability of effectively searching pilot signal window sizes smaller than the pilot signal window size lower limit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 08,416,053,filed on Apr. 4, 1995 U.S. Pat. No. 5,627,835.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of radiocommunication, and more specifically, to the field of pilot signalsearch window control in code division multiple access (CDMA) cellulartelephones.

The primary standard specification relevant to the present invention isTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard forDual-Mode Wideband Spread Spectrum Cellular System. This industrystandard specification is considered understood by those reasonablyskilled in the art of the present invention. Specific sections ofrelevance include all of section 6.6, with particular emphasis onsections 6.6.6.1-6.6.6.2.3, and section 7.7.2.3.2.1.

After a mobile station acquires a pilot signal from a base station, thebase station continually instructs the mobile station to search atspecific locations in time for pilot channel CDMA energy from both thepresent base station and other base stations. Other energy from thepresent base station is important since CDMA systems are able todiversity combine multipath reflections of the same signal into usableenergy through the use of multiple digital receivers, often referred toas demodulating "fingers". In addition, information about the timelocation of other base stations is important from a cellularsoft-handoff standpoint. Besides providing the specific locations tosearch for pilot signal energy, the base station also provides themobile station a search window size for each pilot signal. The availablesearch window sizes range from 4 chips up to 452 chips. Thus, the timerange around each specific time location is defined by the base stationas a search window size.

One physical implementation of CDMA searching mechanisms includes usinga mobile station modem application specific integrated circuit (MSMASIC) and a central processing unit (CPU) with access to memory and adirect memory access controller (DMA controller). Such an MSM ASICincludes a searcher receiver and multiple digital receivers, all ofwhich are controlled through control registers accessible by the CPU. Asa pilot search is performed, result data is stored directly into the CPUmemory through the DMA controller. After the search is complete, aninterrupt is generated which causes the CPU to analyze the result dataand re-assign the digital receivers as necessary. Additionally, a searchfor the next pilot signal is also initiated.

Unfortunately, one of the problems associated with such animplementation relates to CPU processing availability. When searchwindows are small, the CPU is more often required to respond to searchcompletion interrupts and analyze result data. Thus, this process canutilize too much CPU processing time and resources. One way ofaddressing the problem of having too many search completion interruptswould be to simply override the recommended search size by requiringthat all searches be larger than a predefined amount. However, simplyoverriding the search window sizes recommended by the base station woulddefeat the reasons for having small search windows. One of thesepurposes is to restrict each search to isolating a single pilot signal.Thus, for areas with high densities of base stations, it would bedifficult to prevent interference between the base stations without thebenefit of smaller search windows. Another way of addressing the problemwould be to introduce a delay between searches. Unfortunately,implementation of a timer to accomplish such a delay would also add anextra degree of complexity to CPU operation.

There is, therefore, a need in the industry for a method and anapparatus for addressing these and other related, and unrelated,problems.

SUMMARY OF THE INVENTION

A method and an apparatus for searching code division multiple accesspilot signal energies includes limiting a pilot signal window size abovea lower limit and only analyzing search result data corresponding to aninstructed pilot signal window size. A central processing unit instructsa searcher receiver within a mobile station modem application specificintegrated circuit to search for pilot signal energies within aninstructed pilot signal search window unless the instructed pilot signalwindow size is smaller than a pilot signal window size lower limit. Insuch a case, the pilot signal search window size is artificiallydesignated as the pilot signal window size lower limit to preventexcessive search completion interrupts from over utilizing centralprocessing unit resources. The central processing unit analyzes only themiddle search results corresponding to the pilot signal window size asinstructed from the base station to maintain the ability of effectivelysearching pilot signal window sizes smaller than the pilot signal windowsize lower limit.

It is, therefore, an object of the present invention to provide a methodand an apparatus for limiting a pilot signal window size above a lowerlimit and only analyzing search result data corresponding to aninstructed pilot signal window size.

Another object of the present invention is to provide a system forreducing search completion interrupts by overriding pilot signal searchwindow size instruction information from a base station.

Yet another object of the present invention is to provide a system forsimultaneously searching for spread spectrum pilot signal energy andanalyzing search results of a previous search which minimizes processingresources.

Still another object of the present invention is to provide a processfor sequentially searching for pilot signal energies which includes awindow size lower limit.

Still another object of the present invention is to provide a CDMAcellular telephone which limits pilot signal search window sizes tobeing above a defined lower limit regardless of contrary instructionsfrom a base station and which discards data outside a middle rangecorresponding to an instructed smaller window size.

Other objects, features and advantages of the present invention willbecome apparent upon reading and understanding the presentspecification, when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram representation of receiver portions of a CDMAtelephone in accordance with the preferred embodiment of the presentinvention.

FIG. 2 is a flow chart representation of a pilot signal search controlprocess for the elements shown in FIG. 1.

FIG. 3 is a flow chart representation of a pilot signal search analysisprocess for the elements shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in greater detail to the drawings in which like numeralsrepresent like components throughout the several views, FIG. 1 shows anantenna 20 of a code division multiple access (CDMA) cellular telephonefor receiving a spread spectrum cellular radio signal from a basestation. A duplexer 22 directs the received signal into an analogcircuit 24, a baseband application specific integrated circuit (ASIC)26, and a mobile station modem (MSM) ASIC 30, for which only selectedreceiver portions are represented in FIG. 1. As would be understood bythose skilled in the art of the present invention, the analog circuit 24provides conventional analog functionality, such as radiofrequency-to-intermediate frequency downconverting, filtering, andautomatic gain control functions, and the baseband ASIC 26 providesconventional baseband signal processing, including analog-to-digitalconversion.

As the MSM ASIC 30 receives the digital received data stream from thebaseband ASIC 26, it is analyzed by a searcher receiver 34 and threedigital receivers 40, 42, and 44. As configured by settings in controlregisters 46, the digital receivers 40, 42, and 44, also knows as"demodulating fingers", demodulate multi-path information which issubsequently combined into a combined signal by a diversity combiner 50before being de-interleaved by a de-interleaver 52 and decoded by adecoder 54 before being output to a CPU 60. Voice information includedin the information output from the MSM ASIC 30 is then output to avocoder 70 which vocodes the data and provides the vocoded informationto a digital-to-analog converter and speaker 72 for audible output.

As discussed in greater detail below, control information included inthe information output from the MSM ASIC 30 to the CPU 60 is used by theCPU 60 to, in part, configure the control registers 46. This controlregister 46 information is used by the searcher receiver 34 whichsearches for pilot signal energies and, after a search is complete,outputs search result information through a searcher receiver outputchannel 80 to a specific section of the CPU 60 called a direct memoryaccess (DMA) controller 82. Through the DMA controller 82, the searchresult information is stored in memory 90. A result buffer 94 is definedin the memory 90 for storing the search result information from the DMAcontroller 82. Another area of memory 90 shown as window configurationmemory area 96 is also defined to store sizes and locations of pilotsignal search windows as instructed from the base station.

The following describes examples of acceptable elements in accordancewith the preferred embodiment of the present invention. Except for theinternal configuration modifications discussed herein (programming,etc.), prior art examples of an acceptable CPU 60, MSM ASIC 30, andbaseband ASIC 26, and MSM ASIC 30 are, respectively, the 80C186microprocessor available from Advanced Micro Devices of Sunnyvale,Calif., the Q53101-1S2 baseband ASIC available from Qualcomm, Inc. ofSan Diego, Calif., and the Q52501-1S2 MSM also available from Qualcomm,Inc.

Refer now also to FIG. 2 for a flow chart representation of a pilotsignal search control process for the elements shown in FIG. 1. Afterthe process starts at step 100, the CPU 82 reads the windowconfiguration memory area 96 in step 102 to determine the instructedconfiguration (location and size) for the next pilot signal searchwindow. As would be understood by one reasonably skilled in the art ofthe present invention, the location and size for each pilot signalsearch window are denoted in terms of pilot pseudonoise sequence offsetsmeasured in chips and are continually updated through instructionsreceived from the base station. If the instructed pilot signal windowsize of the current search window is not below a pilot signal searchwindow lower limit, e.g., 60 chips, the NO branch of decision block 106is taken to step 108 where, in a conventional manner, the controlregisters 46 are configured to search according to the storedinstruction configuration information. However, if the current searchwindow is instructed to have a size below the pilot signal search windowlower limit, the YES branch of decision block 106 directs operation tostep 110.

In step 110, the CPU 60 configures the control registers 46 of the MSMASIC 30 for a pilot signal search window having the size of the pilotsignal search window lower limit, e.g., 60 chips, centered around theinstructed window configuration. Thus, for example, if the instructedpilot signal search window size is 50 chips, the new size for theartificial pilot signal search becomes 60 chips, adding 5 chips to theupper end and 5 chips to the lower end of the 50 chip size originallyinstructed. Subsequently, after either step 110 or step 108, the MSMASIC 30 is instructed to begin the pilot signal search process in step112. As would be understood by one reasonably skilled in the art of thepresent invention, the searching process includes determining a list ofenergy hypotheses at distinct chip intervals and half intervals over thesearch window. When the search is complete, this information is storedin the result buffer 94, and a search completion interrupt is generated.Until that time, other processing occupies the CPU 60 as indicated instep 114. Furthermore, after the search completion interrupt isreceived, the process of FIG. 2 continues back at step 102 with the nextsearch window.

Refer now also to FIG. 3 for a flow chart representation of a pilotsignal search analysis process for the elements shown in FIG. 1. Afterstarting in step 200 as the result of an initial search completioninterrupt, the instructed window size for the current window is againread in step 202. If the instructed window size is not below the pilotsignal search window lower limit, the NO branch of decision block 206directs operation to step 208 where all of the search result informationfrom the last search stored in the result buffer 94 is analyzed. On theother hand, if the instructed window size is below the pilot signalsearch window lower limit, the YES branch of decision block 206 directsoperation to step 210. In step 210, the CPU 60 analyzes only the searchresult information corresponding to the instructed window size. Thisportion of information is located in the middle of the search resultinformation since the artificial window is centered around theinstructed window. One example of a method for identifying search resultinformation corresponding to the instructed window size for a pilotsignal search window lower limit of 60 chips includes examining therange of memory locations between (60- (instructed window size))/2!through (60+ (instructed window size))/2-1!. In this way, the remaining(outside the middle) search result information created as a result ofthe artificial window enlargement of step 110 in FIG. 2 is ignored, thusaccomplishing the goal of essentially complying with the base stationrequest to utilize data within a specified window without creating alarge burden on the CPU 60. Finally, according to step 212, the CPU 60determines how to assign the digital receivers based upon the searchresults, and other processing continues in step 214 until the nextsearch completion interrupt is detected, at which point control loopsback to step 202 to continually repeat the process of FIG. 3 for thenext search window.

It should be understood that alternate embodiments of the presentinvention include moving functions disclosed herein into an alternateMSM ASIC. Window size comparisons and result data analyses would takeplace internal to such an MSM ASIC. Other alternate embodiments alsoinclude other window size lower limits based upon processor availabilityand speed.

While the embodiments of the present invention which have been disclosedherein are the preferred forms, other embodiments of the presentinvention will suggest themselves to persons skilled in the art in viewof this disclosure. Therefore, it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention and that the scope of the present invention should only belimited by the claims below. Furthermore, the equivalent of all means-or step-plus-function elements in the claims are intended to include anystructure, material, or acts for performing the function as specificallyclaimed and as would be understood by persons skilled in the art of thisdisclosure.

I claim:
 1. A mobile station method of searching for a base stationpilot signal, said method comprising steps of:receiving from a basestation an instructed pilot signal search window size; comparing theinstructed pilot signal search window size to a pilot signal searchwindow size limit, wherein the pilot signal search window size limit isa lower limit; generating a resultant pilot signal search window sizeresponsive to the comparing step, wherein the resultant pilot signalsearch window size is defined to be the pilot signal search window sizelimit responsive to the instructed pilot signal search window size beinglower than the pilot signal search window size limit, and wherein theresultant pilot signal scorch window size is defined to be theinstructed pilot signal search window size responsive to the instructedpilot signal search window size being higher than the pilot signalsearch window size limit: searching for the pilot signal using theresultant pilot signal search window size, including generating resultdata corresponding to the resultant pilot signal search window size;analyzing only a centered middle portion of the result data, wherein theanalyzed portion is determined by {((the pilot signal search window sizelimit)-(the instructed pilot signal search window size))/2} through{((the pilot signal search window size limit)+(the instructed pilotsignal search window size))/2-1}; and assigning a plurality of digitalreceivers based upon the result data.
 2. A mobile station method ofsearching for a base station pilot signal, said method comprising stepsof:receiving from a base station an instructed pilot signal searchwindow size; comparing the instructed pilot signal search window size toa pilot signal search window size limit; generating a resultant pilotsignal search window size responsive to the comparing step; andsearching for the pilot signal using the resultant pilot signal searchwindow size; including generating result data corresponding to theresultant pilot signal search window size; and analyzing only a portionof the result data.
 3. The method as claimed in claim 2, furthercomprising a step of assigning a plurality of digital receivers basedupon the result data.
 4. The method as claimed in claim 2, wherein theanalyzed portion defines a centered middle portion of the result data.5. The method as claimed in claim 2, wherein the analyzed portion isdetermined by{((the pilot signal search window size limit)-(theinstructed pilot signal search window size))/2} through {((the pilotsignal search window size limit)+(the instructed pilot signal searchwindow size))/2-1}.
 6. The method as claimed in claim 2, wherein thepilot signal search window size limit is a lower limit.
 7. The method asclaimed in claim 6, wherein the generating step includes defining theresultant pilot signal search window size to be the pilot signal searchwindow size limit responsive to the instructed pilot signal searchwindow size being lower than the pilot signal search window size limit.8. The method as claimed in claim 7, wherein the generating stepincludes defining the resultant pilot signal search window size to bethe instructed pilot signal search window size responsive to theinstructed pilot signal search window size being higher than the pilotsignal search window size limit.
 9. The method as claimed in claim 6,wherein the pilot signal search window size limit equals at least 60chips.
 10. A mobile station method of searching for a base station pilotsignal, the method comprising steps of:receiving from a base station aninstructed pilot signal search window size; comparing the instructedpilot signal search window size to a pilot signal search window sizelower limit; and searching for the pilot signal using a search windowsize equal to the pilot signal search window size lower limit responsiveto determining the instructed pilot signal search window size to bebelow the pilot signal search window size lower limit.
 11. The method asclaimed in claim 10, further comprising steps of generating searchresult data in search result data memory locations and analyzing only aportion of the search result data.
 12. The method as claimed in claim11, wherein the analyzing step includes analyzing search result datacorresponding only to the instructed pilot signal search window size.13. The method as claimed in claim 11, wherein the analyzing stepincludes examining a range of search result data memory locations as afunction of the pilot signal search window size lower limit and theinstructed pilot signal search window size.
 14. The method as claimed inclaim 13, wherein the range of search result data memory locationsdefine a centered middle portion relative to all of the search resultdata.
 15. The method as claimed in claim 13, wherein the functionincludes{((the pilot signal search window size lower limit)-(theinstructed pilot signal search window size))/2} through {((the pilotsignal search window size lower limit)+(the instructed pilot signalsearch window size))/2-1}.
 16. The method as claimed in claim 10,further comprising a step of assigning digital receivers based upon thesearch result data.
 17. The method as claimed in claim 10, wherein thepilot signal search window size lower limit is based upon mobile stationprocessor availability and speed.